Muscle contraction involves a delicate coordination of neuronally-induced muscle surface membrane depolarization with internal release of internal stores of calcium ion. In between these two processes (excitation and contraction, EC) lies a largely unknown process called EC-coupling which involves an internal membrane system contiguous with the external plasma membrane. Transverse tubular (TT) membranes are anastomosing invaginations of the plasma membrane (or sarcolemma, SL) of the striated muscle cell and are considered to be "surface" membrane; they comprise over 80% of the total surface membrane of skeletal muscle. As the TT membranes invaginate towards the interior of the cell, they periodically contact the terminal cisternae of the sarcoplasmic reticulum (SR). Because of this unique structural arrangement, TT membranes initiate the release of calcium which is prerequisite to muscle contraction. However, the general understanding of this process is poor because direct evidence for the mechanism of TT- induced SR calcium release requires information derived from the characterization of purified TT membranes. Our approach to the problem has been to isolate TT membranes from heterogenous microsomal fractions of muscle, characterize them structurally and biochemically and interpret our data in terms of proposed mechanisms of EC coupling. The proposed research focusses upon a newly discovered ATPase localized within the TT membrane, the TT-MgATPase. Studies are described which involve the cloning and sequencing of this novel enzyme, the determination of its topological orientation within the TT, the exploration of its functional role in EC coupling or relaxation, and various aspects of its structure and regulatory phenomena. Importantly, this enzyme is the second example of diacylglycerol-regulated (DAG) enzyme. In contrast to protein kinase c, the TT-MgATPase is inhibited by phorbol esters and DAG's, and the ATPase is the predominant DAG receptor in muscle. Several other experimental topics involving TT membranes are discussed. Students will be able to select among several research topics.